1. Field of the Invention
The present invention relates to a soft magnetic alloy sheet having low residual magnetic flux density, which is used as a core of distribution transformers, electric power and industrial equipment transformers, direct current sensors, and current transformers.
2. Description of the Related Arts
Generally, distribution transformers use grain-oriented silicon steel sheets because that type of sheets allow the design of high magnetic flux density while suppressing the iron loss to a low level. The grain-oriented silicon steel sheets have, however, a drawback of residual induction owing to their high residual magnetic flux density. When residual induction exists, distribution transformers in buildings and in an environment that widely uses inverter power source may induce overcurrent in case of power failure or in case of reclosing of power because of the saturation of magnetic flux, and may finally result in the occurrence of iron loss in the power source equipment of power distribution system and further the generation of serious damages on other power system. To prevent such defects, distribution transformers are designed to reduce residual magnetic flux density by placing a gap in magnetic path to avoid the occurrence of residual induction.
Owing to the design, the characteristic of high magnetic flux density which is an inherent feature of grain-oriented silicon steel sheets cannot be utilized, and the transformer becomes large. In addition, existence of gap increases iron loss at the gap portion.
Direct current sensors have a gap in magnetic path, and detects the magnetic flux crossing the gap. Also the direct current sensors have similar problem as the distribution transformers have. That is, owing to the high residual magnetic flux density in core, the sensor cannot function in detecting current during a period of decreasing the current from a high level to a low level because of the residual magnetism in the core.
For power and transmission current transformers, cutting may be applied thereto for preventing error in evaluation of break of transmission line induced by the occurrence of residual induction under accidental overcurrent resulted from lightening or the like.